Polyphenylene ether resins are well known in the art as a class of thermoplastics which possess a number of outstanding physical properties. They can be prepared by oxidative and non-oxidative methods, such as are disclosed, for example, in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358.
It has been found that many of the properties of polyphenylene ether resins, e.g., ease of processing, impact strength and solvent resistance, can be improved by combining these resins with other resins, such as, for example, polystyrene or copolymers of polystyrene. Examples of polyphenylene ether resin-polystyrene compositions are disclosed in Cizek, U.S. Pat. No. 3,383,434.
More recently, it has been found that polyphenylene ether resins can also be combined with block copolymers of the A-B-A.sup.1 type, e.g., polystyrene-polybutadiene-polystyrene, and with acrylic resin modified diene rubber containing resins, to provide compatible compositions characterized by a number of excellent physical properties in the resulting molded articles.
These discoveries are described in Abolins et al., U.S. Pat. Nos. 3,833,688 and 3,792,123 and in Haaf U.S. Pat. No. 4,167,507, which is assigned to the same assignee as the present application.
It is known in the art that the polyphenylene ethers have excellent flame retardant properties and are classified self-extinguishing and non-dripping according to ASTM Test Method D635 and Underwriters Laboratories Bulletin No. 94. On the other hand, when polyphenylene ethers are combined with other polymers such as the above-mentioned A-B-A.sup.1 block copolymers, many of the resulting compositions have poor flame retardancy and are not self-extinguishing, but rather burn slowly upon ignition. Consequently, many compositions composed of polyphenylene ether resin and A-B-A.sup.1 block copolymer resins are unable to meet the minimum requirements established by various testing laboratories such as the Underwriters Laboratories. This restricts the use of such compositions for many commercial applications.
Flame retardant additives for thermoplatics are known. In general, these are either blended physically with the thermoplastic or are used to unite chemically with the plastic and to modify it. For instance, self-extinguishing blends of a polyphenylene ether resin and a styrene resin using a combination of an aromatic phosphate and an aromatic halogen for flame retardancy are disclosed by Haaf in U.S. Pat. No. 3,639,506. Other self-extinguishing polyphenylene ether-polystyrene compositions are disclosed by Reinhard in U.S. Pat. No. 3,809,729, wherein aromatic halogens combined with antimony compounds are used as flame retardant additives. Still other flame retardant compositions of a polyphenylene ether resin and a styrene resin which include various phosphorus-containing and halogen-containing flame retardant agents, are described by Haaf et al. in U.S. Ser. No. 647,981, filed Dec. 27, 1975, and assigned to the same assignee as the present application.
However, as is also well known, the inclusion of flame retarding compounds in thermoplastic materials not only affects burning characteristics, it frequently changes other physical properties as well, such as color, flexibility, tensile strength, electrical properties, softening point, and moldability characteristics. Thus, for example, aromatic phosphates such a triphenyl phospate have been added to blends of polyphenylene ethers and styrene ethers, with flame retardant properties being improved to the point where the compositions can be classified as self-extinguishing and non-dripping according to the above-noted ASTM Test Method D635 and U.L. Bulletin No. 94.
It has also been found that molded compositions consisting of polyphenylene ether or various combinations of polyphenylene ether and certain polymeric modifiers, and aromatic phosphate compounds exhibit excellent self-extinguishing behavior, impact strength and acceptable surface gloss. Such compositions are disclosed by Haaf et al. in co-pending application Ser. No. 870,894, filed Jan. 20, 1978, and assigned to the same assignee as the present invention.
As can be seen by the foregoing, various compositions employing polyphenylene ether resin in admixture with a variety of plasticizer compositions and impact modifiers have been disclosed. It has, nonetheless, now been surprisingly discovered that thermoplastic molding compositions having a combination of improved melt-flow characteristics and, after molding, exceptional impact resistance, may be obtained by the utilization of a polyphenylene ether resin in admixture with a plasticizer composition and a minor amount of a particular class of radical teleblock copolymers, those being the hydrogenated radial teleblock copolymers of a vinyl aromatic copolymer and a saturated rubber, as will become apparent from the following description of the invention.